The present invention relates to a solid supported catalyst usable for the polymerization of conjugated dienes, to a process for the preparation of said catalyst and to a process for the polymerization of conjugated dienes using said catalyst. The present invention applies, for example, to a catalyst comprising a solid, silica-based support.
The person skilled in the art is thoroughly familiar with the use of rare earth compounds for the preparation of catalysts intended for polymerizing conjugated dienes. An essential condition for satisfactory activity of the resultant catalyst is that the rare earth compound be sufficiently well dispersed in the polymerization solvent.
European patent EP-A-856 530 discloses the use of organic rare earth derivatives for preparing a solid supported catalyst. These organic derivatives are known to be soluble in hydrocarbon polymerization solvents and, consequently, provide a good dispersion of the corresponding rare earth in the solvent.
The catalyst described in said document is prepared by reacting a solid, silica-based inorganic support in such a solvent                on the one hand, with an alkylaluminum chloride, then with a solution comprising an organic rare earth derivative, such as a neodymium carboxylate or,        on the other hand, with a co-catalyst consisting of an alkylaluminum or an alkylaluminum hydride.        
One major disadvantage of this catalyst is the necessity of previously synthesising the organic rare earth derivative for the purpose of preparing said catalyst.
European patent EP-A-637 589 describes homogeneous, i.e. unsupported, catalysts based on a rare earth halide which are usable for polymerizing conjugated dienes. These catalysts are of the formula M(Ar)(AlX3R)3 and are prepared by reacting exactly 1.5 equivalents of a trialkylaluminum with a complex of the formula M(Ar)(AlX4)3 (in these formulae, M is a rare earth such as neodymium, Ar is an aromatic hydrocarbon solvent, such as toluene, Al an atom of aluminum and X a halogen).
Reference may be made to the article by H. Liang, Q. Shen, J. Guan et Y. Lin, Journal of Organometallic Chemistry, 474 (1994), pp. 113–116, for details of the method for preparing such a complex of the formula M(Ar)(AlX4)3.
One major disadvantage of this homogeneous catalyst is that the activity thereof is very highly dependent upon the aluminum: rare earth ratio.
Thus, in the presence of a large excess of trialkylaluminum, the homogeneous catalyst of the above formula M(Ar)(AlX3R)3 is not obtained, but rather a complex of the formula M(Ar)(AlX4)3 is obtained, which dissociates to form a rare earth trihalide of the formula MX3 (c.f article in New. J. Chem. (1995) 19, pp. 713–722).
Following the example of EP-A-637 589, the article of Hu Jing-Yu, Tian He-Qin, Shen Qi, et Liang Hong-Ze, Chinese Science Bulletin, Vol. 37, no. 7, pp. 566–570 (1992) describes a homogeneous catalyst for polymerizing isoprene. This unsupported catalyst is obtained by reacting a complex of the formula Nd(C6H6)(AlCl4)3 with a trialkylaluminum (where Nd is neodymium and C6H6 is benzene).
This catalyst differs from that described in EP-A-637 589 by the possibility of using aluminum:neodymium ratios which may reach higher values, such as 30 or 40, in order to obtain a catalyst having satisfactory catalytic activity during the isoprene polymerization reaction.
However, one major disadvantage of this homogeneous catalyst resides in reduced viscosity values and the level of cis-1,4 linkages which it brings about in the resultant polyisoprenes.
The object of the present invention is to overcome these disadvantages.